U.S. patent number 7,270,764 [Application Number 11/171,686] was granted by the patent office on 2007-09-18 for method for removing aluminide coating from metal substrate and turbine engine part so treated.
This patent grant is currently assigned to General Electric Company. Invention is credited to William Clarke Brooks, Brian H. Pilsner, James Douglas Risbeck, Mark Alan Rosenzweig, Richard Roy Worthing, Jr., Roger Dale Wustman.
United States Patent |
7,270,764 |
Wustman , et al. |
September 18, 2007 |
Method for removing aluminide coating from metal substrate and
turbine engine part so treated
Abstract
A method for selectively removing an aluminide coating from at
least one surface of a metal-based substrate by: (a) contacting the
surface of the substrate with at least one stripping composition
comprising nitric acid at a temperature less than about 20.degree.
C. to degrade the coating without damaging the substrate; and (b)
removing the degraded coating without damaging the substrate. Also
disclosed is a method for replacing a worn or damaged aluminide
coating on at least one surface of a metal-based substrate by
selectively removing the coating using the above steps, and then
applying a new aluminide coating to the surface of the substrate.
Turbine engine parts, such as high-pressure turbine blades, treated
using the above methods are also disclosed.
Inventors: |
Wustman; Roger Dale (Mason,
OH), Rosenzweig; Mark Alan (Hamilton, OH), Brooks;
William Clarke (Lebanon, OH), Pilsner; Brian H. (Mason,
OH), Risbeck; James Douglas (Cincinnati, OH), Worthing,
Jr.; Richard Roy (Cincinnati, OH) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
32507465 |
Appl.
No.: |
11/171,686 |
Filed: |
June 30, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050244274 A1 |
Nov 3, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10339475 |
Jan 9, 2003 |
7008553 |
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Current U.S.
Class: |
216/108;
29/889.1; 134/41 |
Current CPC
Class: |
C23F
1/44 (20130101); C23F 1/30 (20130101); F01D
5/005 (20130101); F05D 2300/611 (20130101); Y02T
50/60 (20130101); Y10T 29/49318 (20150115); F05D
2230/90 (20130101); F05D 2230/80 (20130101) |
Current International
Class: |
B44C
1/22 (20060101); B23P 6/00 (20060101); C03C
15/00 (20060101); C03C 25/68 (20060101); C23F
1/00 (20060101) |
Field of
Search: |
;216/103,108,41
;134/3,41 ;29/889.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 136 593 |
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Sep 2001 |
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EP |
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1 525 322 |
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Sep 1978 |
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GB |
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1 565 107 |
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Jun 1980 |
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GB |
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Primary Examiner: Culbert; Roberts
Attorney, Agent or Firm: Hasse; Donald E. Nesbitt; Daniel F.
Hasse & Nesbitt LLC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser.
No. 10/339,475, filed on Jan. 9, 2003 now U.S. Pat. No. 7,008,553,
incorporated herein by reference.
Claims
What is claimed:
1. A used turbine engine blade having a metal-based substrate and
an aluminide coating on at least one internal surface of the
airfoil portion of the blade and at least one external surface of
the airfoil portion of the blade, and wherein at least a portion of
an aluminide coating has been selectively removed from at least one
internal surface of the root and shank portions of the blade with
negligible effect on the aluminide coating on the internal and
external surfaces of the airfoil portion of the blade by a method
comprising the following steps: (a) contacting the at least one
internal surface of the root and shank portions of the blade with
at least one stripping composition comprising from about 20% to
about 40% nitric acid, by weight of the composition, at a
temperature less than about 20.degree. C. for at least about 2
hours to degrade the coating without damaging the substrate; and
(b) removing the degraded coating without damaging the
substrate.
2. The turbine engine part of claim 1, wherein the aluminide
coating comprises at least one compound selected from the group
consisting of aluminide, platinum aluminide, nickel aluminide,
platinum-nickel aluminide, refractory-doped aluminides, and alloys
comprising at least one of the foregoing.
3. The turbine engine part of claim 2, wherein the metal-based
substrate is a nickel-based superalloy or a cobalt-based
superalloy.
4. A used turbine engine blade having a metal-based substrate and
an aluminide coating on at least one internal surface of the
airfoil portion of the blade and at least one external surface of
the airfoil portion of the blade, and wherein at least a portion of
an aluminide coating has been selectively removed from at least one
internal surface of the root and shank portions of the blade with
negligible effect on the aluminide coating on the internal and
external surfaces of the airfoil portion of the blade by a method
comprising the following steps in sequence: (a) contacting the at
least one internal surface of the root and shank portions of the
blade with an aqueous solution comprising from about 10% to about
50%, by weight of the solution, of caustic at a temperature of from
about 60.degree. C. to about 100.degree. C. for from about 20
minutes to about 4 hours; (b) contacting the at least one internal
surface of the root and shank portions of the blade with at least
one stripping composition comprising from about 20% to about 40%
nitric acid, by weight of the composition, at a temperature less
than about 20.degree. C. for at least about 2 hours to degrade the
coating without damaging the substrate; and (c) removing the
degraded coating without damaging the substrate.
5. The turbine engine part of claim 4, wherein the stripping
composition comprises from about 25% to about 35% nitric acid, by
weight of the composition.
6. The turbine engine part of claim 4, wherein the stripping
composition has a temperature of from about 0.degree. C. to about
15.degree. C.
7. The turbine engine part of claim 4, wherein the stripping
composition contacts the substrate for a period of from about 4
hours to about 10 hours.
8. The turbine engine part of claim 4, wherein the stripping
composition contacts the substrate for a period of from about 6
hours to about 8 hours.
9. The turbine engine part of claim 4, wherein the stripping
composition comprises from about 25% to about 35% nitric acid, by
weight of the composition, and has a temperature of from about
4.degree. C. to about 12.degree. C.
10. The turbine engine part of claim 4, wherein the stripping
composition further comprises from about 0.1% to about 5%, by
weight of the composition, of a wetting agent.
11. The turbine engine part of claim 4, wherein the stripping
composition further comprises from about 0.1% to about 5%, by
weight of the composition, of a wetting agent selected from the
group consisting of polyalkylene glycols, glycerol, fatty acids,
soaps, emulsifiers, and surfactants.
12. The turbine engine part of claim 11, wherein the root and shank
portions of the blade are immersed in a bath of the stripping
composition in step (b) for a period of from about 6 hours to about
8 hours.
13. The turbine engine part of claim 4, wherein the root and shank
portions of the blade are immersed in a bath of the stripping
composition in step (b).
14. The turbine engine part of claim 13, wherein the bath is
maintained at a temperature of from about 0.degree. C. to about
15.degree. C. while the root and shank portions of the blade are
immersed therein for a period of from about 4 hours to about 10
hours.
15. A turbine engine blade used in service in a commercial gas
turbine engine and having a metal-based substrate and an aluminide
coating on at least one internal surface of the airfoil portion of
the blade and on at least one external surface of the airfoil
portion of the blade, in which a worn or damaged aluminide coating
on at least one internal surface of the root and shank portions of
the blade has been replaced by a method comprising the following
steps in sequence: (i) selectively removing the aluminide coating
from the at least one internal surface of the root and shank
portions of the blade with negligible effect on the aluminide
coating on the internal and external surfaces of the airfoil
portion of the blade by (a) contacting the surface with an aqueous
solution comprising from about 10% to about 50%, by weight of the
solution, of caustic at a temperature of from about 60.degree. C.
to about 100.degree. C. for from about 20 minutes to about 4 hours;
(b) contacting the surface with at least one stripping composition
comprising from about 20% to about 40% nitric acid, by weight of
the composition, at a temperature of less than about 20.degree. C.
for at least about 2 hours to degrade the coating without damaging
the substrate; and (c) removing the degraded coating without
damaging the substrate; and (ii) applying a new aluminide coating
to the at least one internal surface of the root and shank portions
of the blade.
16. The turbine engine part of claim 15, wherein the stripping
composition comprises from about 25% to about 35% nitric acid, by
weight of the composition.
17. The turbine engine part of claim 15, wherein the stripping
composition has a temperature of from about 0.degree. C. to about
15.degree. C.
18. The turbine engine part of claim 15, wherein the stripping
composition contacts the surface for a period of from about 4 hours
to about 10 hours.
19. The turbine engine part of claim 15, wherein the caustic
solution comprises from about 15% to about 30%, by weight, of
potassium hydroxide.
20. The turbine engine part of claim 19, wherein the stripping
composition comprises from about 28% to about 32% nitric acid, and
the root and shank portions of the blade are immersed in a bath of
the stripping composition having a temperature of from about
4.degree. C. to about 12.degree. C. for from about 6 hours to about
8 hours.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method for removing an aluminide
coating from a metal-based substrate. More particularly, the
invention is directed to a method for selectively removing an
aluminide coating by using a stripping composition to degrade the
coating and then removing it without damaging the substrate. The
invention also relates to a turbine engine part having an aluminide
coating, at least a portion of which has been selectively removed
by the above method.
A variety of coatings are often used to protect metal parts exposed
to high temperatures, such as parts made from superalloys. For
example, gas turbine engine components (and other industrial parts)
are often formed of superalloys that can withstand a variety of
extreme operating conditions. Such parts are usually covered with
coatings to protect them from environmental degradation, including
the adverse effects of corrosion and oxidation. Coatings used on
components in gas turbine hot sections, such as blades, nozzles,
combustors, turbine shrouds and transition pieces, generally belong
to one of two classes: diffusion coatings or overlay coatings.
Diffusion coatings are typically formed of aluminide-type alloys,
such as nickel-aluminide; a noble metal-aluminide such as
platinum-aluminide; or nickel-platinum-aluminide. Overlay coatings
typically have the composition MCrAl(X), where M is an element
selected from the group consisting of Ni, Co, Fe, and combinations
thereof, and X is an element selected from the group consisting of
Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Diffusion
coatings are formed by depositing constituent components of the
coating, and reacting those components with elements from the
underlying substrate, to form the coating by high temperature
diffusion. In contrast, overlay coatings are generally deposited
intact, without reaction with the underlying substrate.
During service, diffusion and overlay coatings are often exposed to
oxidative conditions. For example, coatings on turbine airfoils are
typically subjected to oxidation in the hot gas path during normal
operation. Under such conditions, with temperatures in the range of
about 525-1150.degree. C., various oxidative products are formed on
the coatings. For example, aluminum oxides and other metal oxides,
including nickel oxide, cobalt oxide, chromium oxide, and other
base metal oxides, often form on simple aluminide and
platinum-aluminide coatings. Aluminum oxides, chromium oxides, and
various spinels often form on the MCrAl(X)-type coatings.
When turbine engine components are overhauled, the protective
coatings are often removed to allow inspection and repair of the
underlying substrate. Various stripping compositions have been used
to remove the coatings. Usually, the oxide materials must be
removed before the coatings can be treated with the stripping
composition. Various techniques have been used for oxide removal.
For example, oxide materials often have been removed from external
sections of turbine components by grit blasting.
Alternatively, turbine components have sometimes been treated in an
oxide-removal solution comprising a strong mineral acid or a strong
caustic. Examples of such mineral acids are hydrochloric acid,
sulfuric acid, and nitric acid. The caustic solutions usually
include sodium hydroxide, potassium hydroxide, or various molten
salts. Repeated treatments sometimes are used to remove the oxide.
After removal of the oxide is completed, the substrate is then
typically immersed in another solution suitable for removing the
coating material itself. In current practice, the aluminide
materials are often stripped from the substrate by exposure to
various acids or combinations of acids, e.g., hydrochloric acid,
nitric acid, and phosphoric acid.
There are some drawbacks associated with the use of the various
stripping compositions mentioned above. Some stripping compositions
do not remove sufficient amounts of the aluminide material. Other
compositions that remove the aluminides also attack the base metal
of the substrate, pitting the base metal or damaging the metal via
intergranular or interdendritic (in the case of single crystal
materials) attack. Some stripping compositions are used at elevated
temperatures, e.g., above about 75.degree. C. to speed the reaction
and removal of the coating. Operation at these temperatures can
promote increased attack of the base metal and may require masking
materials to protect selected portions of the metal part, e.g.,
airfoil internal surfaces. Elevated temperature processes also
increase energy costs and potentially require additional safety
precautions. Airfoil internal surfaces are often filled with wax or
plastic to protect surfaces that do not require stripping. These
materials must be removed before using the part, adding additional
manufacturing steps and cost. Moreover, conventional treatment
solutions that employ large quantities of strong mineral acids may
emit an excessive amount of hazardous fumes that must be scrubbed
from ventilation exhaust systems.
Some processes use grit-blasting prior to acid treatment to
pretreat and activate the substrate surface, and after exposure to
the stripping composition to remove residual degraded coating.
These steps can be very time-consuming, and can also damage the
substrate and limit part life. Special care may need to be taken to
prevent grit-blasting damage to the substrate or any protective
coating not being removed from the metal part. Moreover,
grit-blasting cannot generally be used to remove oxide material
from internal passages or cavities in metal parts. For example,
grit-blasting would not be suitable for use in the internal cooling
passages of high pressure turbine blades where the grit particles
could block the internal passages.
It is thus apparent that new processes for removing aluminide
coatings from metal substrates would be welcome in the art. It
would be desirable if the processes remove substantially all of the
aluminide coating, while not damaging the base metal. Moreover, it
would be desirable if the processes could be carried out at lower
temperatures to minimize or eliminate base metal attack. It would
also be desirable if the processes eliminate preliminary steps like
grit-blasting, so that they can be used to effectively remove
coatings from internal sections of metal parts without blocking
internal passages.
BRIEF DESCRIPTION OF THE INVENTION
In one aspect, this invention relates to a method for selectively
removing an aluminide coating from at least one surface of a
metal-based substrate, comprising the following steps: (a)
contacting the surface of the substrate with at least one stripping
composition comprising from about 20% to about 40% nitric acid, by
weight of the composition, at a temperature less than about
20.degree. C. for at least about 2 hours to degrade the coating
without damaging the substrate; and (b) removing the degraded
coating without damaging the substrate.
As used herein, "selective removal" of the aluminide coating refers
to the removal of a relatively large percentage of the aluminide
material while removing only a very small portion (or none) of the
substrate material. Any affected portion of the metal substrate is
usually to a depth of less than about 0.0015 inches (about 38
microns), typically less than about 0.001 inches (about 25
microns), and more typically less than about 0.0005 inches (about
13 microns).
The term "aluminide material" in this context is meant to include a
variety of materials typically used in coating metal alloys
(especially superalloys), or which are formed during or after the
coating process. Non-limiting examples include simple aluminide,
platinum-modified aluminide, nickel aluminide, cobalt aluminide,
platinum-nickel aluminide, refractory-doped aluminide, or alloys
comprising one or more of those compounds.
In another aspect, this invention relates to a method for
selectively removing an aluminide coating from at least one
internal surface of a metal-based substrate, comprising the
following steps: (a) contacting the surface of the substrate with
an aqueous solution comprising from about 10% to about 50%, by
weight of the solution, of caustic at a temperature of from about
60.degree. C. to about 100.degree. C. for from about 1 to about 4
hours; (b) contacting the surface of the substrate with at least
one stripping composition comprising from about 20% to about 40%
nitric acid, by weight of the composition, at a temperature less
than about 20.degree. C. for at least about 2 hours to degrade the
coating without damaging the substrate; (c) contacting the surface
of the substrate with an aqueous solution comprising from about 10%
to about 50%, by weight of the solution, of caustic at a
temperature of from about 60.degree. C. to about 100.degree. C. for
from about 20 minutes to about 2 hours; (d) contacting the surface
of the substrate with at least one stripping composition comprising
from about 20% to about 40% nitric acid, by weight of the
composition, at a temperature less than about 20.degree. C. for at
least about 2 hours to degrade the coating without damaging the
substrate; and (e) contacting the surface of the substrate with an
aqueous solution comprising from about 10% to about 50%, by weight
of the solution, of caustic at a temperature of from about
60.degree. C. to about 100.degree. C. for from about 1 to about 4
hours to remove the degraded coating without damaging the
substrate.
In another aspect, the invention relates to a turbine engine part
having a metal-based substrate and an aluminide coating on at least
one surface thereof, at least a portion of which coating has been
selectively removed from the surface of the substrate using the
above methods.
Another aspect of the invention relates to a method for selectively
removing an aluminide coating from at least one internal surface of
a superalloy substrate, comprising the following steps: (a)
contacting the surface of the substrate with at least one stripping
composition comprising from about 25% to about 35% nitric acid, by
weight of the composition, at a temperature of from about 0.degree.
C. to about 15.degree. C. for from about 4 hours to about 10 hours
to degrade the coating without damaging the substrate; and (b)
removing the degraded coating without damaging the substrate.
In yet another aspect, the invention relates to a method for
replacing a worn or damaged aluminide coating on at least one
surface of a metal-based substrate, comprising the steps of
selectively removing the aluminide coating from the surface using
the above methods, and then applying a new aluminide coating to the
surface of the substrate.
Other details regarding the various embodiments of this invention
are provided below.
DETAILED DESCRIPTION OF THE INVENTION
The above methods comprise the step of contacting the surface of
the substrate with at least one stripping composition comprising
from about 20% to about 40% nitric acid, by weight of the
composition, at a temperature less than about 20.degree. C. for at
least about 2 hours to degrade the coating without damaging the
substrate.
Within the above ranges, various stripping compositions and
processing conditions can be used in the process of the invention.
The choice of a particular composition or condition will depend on
various factors, such as the type of substrate, the type of
aluminide coating being removed from the substrate, the intended
end use for the substrate, and the presence or absence of
additional treatment steps (e.g., pretreatment, desmutting,
neutralization and/or rinsing steps). Those skilled in the art will
be able to choose appropriate stripping compositions and processing
conditions for a given situation, based on the teachings
herein.
The substrate of the present invention can be any metallic material
or alloy typically protected by an aluminide coating. As used
herein, "metallic" refers to substrates that are primarily formed
of metal or metal alloys, but which may also include some
non-metallic components. Non-limiting examples of metallic
materials comprise at least one element selected from the group
consisting of iron, cobalt, nickel, aluminum, chromium, titanium,
and mixtures thereof (e.g., stainless steel).
Often, the substrate is a heat-resistant alloy, e.g., a
nickel-based material or cobalt-based material. Such materials are
described in various references, including U.S. Pat. Nos. 5,399,313
and 4,116,723. The type of substrate can vary widely, but it is
often in the form of a jet engine part, such as an airfoil
component. As another example, the substrate may be the piston head
of a diesel engine, or any other substrate requiring a
heat-resistant or oxidation-resistant coating. The substrate may
also be in the form of a houseware item (e.g., cookware), or other
industrial hardware or equipment.
The metallic material is often a superalloy, typically nickel-,
cobalt-, or iron-based, although nickel- and cobalt-based alloys
are favored for high-performance applications. The base element,
typically nickel or cobalt, is the single greatest element in the
superalloy by weight. Nickel-based superalloys usually include at
least about 40% Ni, and at least one component selected from the
group consisting of cobalt, chromium, aluminum, tungsten,
molybdenum, titanium, and iron. Examples of nickel-base superalloys
are designed by the trade names Inconel.RTM., Nimonic.RTM., and
Rene.RTM., and include directionally solidified and single crystal
superalloys. Cobalt-based superalloys usually include at least
about 30% Co, and at least one component from the group consisting
of nickel, chromium, aluminum, tungsten, molybdenum, titanium, and
iron. Examples of cobalt-based superalloys are designated by the
trade names Haynes.RTM., Nozzaloy.RTM., Stellite.RTM. and
Ultimet.RTM..
The aluminide coating on the substrate may be applied in a variety
of locations on a component. In the case of a turbine engine, the
coating is often applied on combustor liners, combustor domes,
shrouds, airfoils, including buckets or blades, nozzles, and vanes.
The coating can be found on the flat areas of substrates, as well
as on curved or irregular surfaces. The coating may also be formed
on the surfaces of internal cavities in the substrates, e.g.,
indentations, hollow regions, or holes. For example, the cavities
can be in the form of radial cooling holes or serpentine
passageways, which can have an overall length of up to about 30
inches (about 76.2 cm) in turbine engine airfoils. It is often
difficult to remove the coating from the surface of these cavities
by conventional, line-of-sight processes such as grit blasting,
plasma etching, or laser ablation.
The thickness of the coating will depend on a variety of factors.
These include the length of service time for the component, its
thermal history, and the particular composition of the coating and
substrate. Usually the coating has a thickness in the range of from
a few microns to about 150 microns, and most often in the range of
from about 25 microns to about 75 microns.
The stripping composition of the present invention comprises from
about 20% to about 40%, typically from about 25% to about 35%, more
typically from about 28% to about 32%, by weight of the
composition, of nitric acid. This relatively high concentration of
nitric acid often causes less base metal attack than lower
concentrations of nitric acid. The balance of the stripping
composition typically is a suitable solvent, such as water,
although minor amounts of other acids and additives as described
below may be included in the composition. Inorganic acids, such as
hydrochloric acid and sulfuric acids, and aliphatic and aromatic
acids useful herein are disclosed in U.S. Pat. No. 5,976,265,
Sangeeta et al.
The stripping composition of the present invention may include
various other additives that serve a variety of functions.
Non-limiting examples of these additives are solvents, inhibitors,
dispersants, surfactants, chelating agents, wetting agents,
deflocculants, stabilizers, anti-settling agents, oxidizing agents,
reducing agents, and anti-foam agents. Those of ordinary skill in
the art are familiar with such additives, and with effective levels
for their use.
In certain embodiments, an organic solvent may be used to reduce
the activity and increase the wetting capability of the nitric acid
relative to the substrate. (The chemical interaction between an
acid and a hydrocarbon solvent will often differ from the
interaction between the acid and a solvent like water.) The
combination of nitric acid and the organic solvent may remove
substantially all of the aluminide coating material without
adversely affecting the substrate. As used herein, "activity"
generally refers to a measurement of the reactivity of the acid
toward the substrate and/or the aluminide coating being removed
from the substrate.
Examples of organic solvents which generally meet the requirements
for this class of stripping compositions are aliphatic alcohols,
aromatic alcohols, chlorinated alcohols, ketones, nitrile-based
solvents, nitrated hydrocarbon solvents, nitrated aromatic solvents
such as nitrobenzene, chlorinated hydrocarbons, amines, and
mixtures of any of the foregoing. Specific examples of aliphatic
alcohols useful herein are methanol, ethanol, and isopropanol.
Mixtures of alcohols may be used as well. Specific examples of
aromatic alcohols are phenols and substituted phenols.
The use of such mixtures may occasionally result in slight pitting,
or in a small amount of corrosion of the substrate, which is
typically substantially uniform. As used herein, "uniform
corrosion" refers to the removal of a very thin, continuous layer
of the substrate, usually less than about 2 microns in thickness.
Uniform corrosion and slight pitting are not a significant drawback
for some end uses of the substrate. This is in contrast to the
occurrence of severe "pitting", which results in holes in the
substrate, often to a depth of at least about 25 microns, and
usually to a depth in the range of from about 25 microns to about
500 microns.
In some embodiments, the stripping composition further includes a
wetting agent. The wetting agent reduces the surface tension of the
composition, permitting better contact with the substrate and the
aluminide coating, particularly on internal surfaces of metal
parts, to improve stripping of the aluminide coating. Suitable
wetting agents include polyalkylene glycols, glycerol, fatty acids,
soaps, emulsifiers, and surfactants. The wetting agent is usually
present at a level in the range of from about 0.1% by weight to
about 5% by weight, based on the total weight of the
composition.
Inhibitors such as acetic acid are sometimes employed in the
stripping composition to lower the activity of the acid in the
composition. The lowered activity in turn decreases the potential
for pitting of the substrate surface. If used, the level of
inhibitor usually is from about 1% by weight to about 15% by
weight, based on the weight of the stripping composition.
Oxidizing agents are sometimes used in the stripping composition to
prevent the formation of a reducing environment. Examples include
peroxides (e.g., hydrogen peroxide), chlorates, perchlorates,
nitrates, permanganates, chromates, and osmates (e.g., osmium
tetroxide). The level of oxidizing agent used is usually from about
0.01% by weight to about 5% by weight, based on the weight of the
entire stripping composition.
The stripping composition may be applied to the substrate in a
variety of ways. In some embodiments, the substrate is immersed,
either partially or fully, in a bath of the composition. Immersion
in this manner (in any type of vessel) often permits the greatest
degree of contact between the composition and the coating being
removed. The substrate may be lowered into the bath using a
suitable rack (for example, one having a polypropylene or other
non-conductive surface) that can be raised to remove the substrate
after the desired immersion time is reached. Immersion time and
bath temperature will depend on many of the factors described
above, such as the type of coating being removed and the acid (or
acids) being used in the bath. However, the bath is typically
maintained at a temperature below about 20.degree. C. while the
substrate is immersed therein. In some embodiments, the bath is
maintained at a temperature of from about 0.degree. C. to about
15.degree. C., often from about 4.degree. C. to about 12.degree. C.
Temperatures much higher than 20.degree. C. typically result in
more rapid removal of the aluminide coating and may cause excessive
pitting of the base metal. Use of the lower temperatures herein
protects the metal substrate and masking materials that may be
present, and also reduces safety hazards associated with
higher-temperature baths when volatile components are present.
Baths comprising the stripping composition are often stirred or
otherwise agitated while the process is carried out, to permit
maximum contact between the stripping agent and the coating being
removed. A variety of known techniques can be used for this
purpose, such as using impellers, ultrasonic agitation, magnetic
agitation, gas bubbling, or circulation-pumps. Immersion time in
the bath will vary based on many of the factors discussed above. On
a commercial scale, the immersion time will usually range from
about 2 hours to about 20 hours in total, which may be split among
two or more stripping steps. In some embodiments, the total
immersion time will be from about 3 to about 15 hours, typically
from about 4 to about 10 hours, more typically from about 6 to
about 8 hours. Longer stripping times within the above ranges
promote more complete removal of the aluminide coating but can
cause greater base metal attack. Thus, the stripping time, the
concentration of nitric acid in the stripping composition, and the
temperature of the stripping composition are selected to provide
the desired balance between maximizing removal of the aluminide
coating and minimizing base metal attack for a particular coating
and metal substrate.
Exposure to the stripping composition causes the aluminide coating
on the surface of the substrate to become degraded. For example,
the coating may have deep cracks, its integrity may be diminished,
and its adhesion to the substrate may be substantially decreased.
In some embodiments, the surface may be rinsed by contact with or
immersion in water or an aqueous solution for a short time, e.g.,
less than about 1 minute, to remove the stripping composition
and/or degraded coating from the surface.
Removal of the degraded coating without damaging the substrate may
be accomplished by various other methods known in the art. For
example, the degraded coating may be removed by abrading the
substrate surface, such as by using a gentle abrasion step that
minimizes damage to the substrate. As an example, light
grit-blasting can be carried out by directing a pressurized air
stream comprising aluminum oxide particles across the surface at a
pressure of less than about 40 psi (about 2.8 kgf/cm.sup.2),
typically less than about 20 psi (about 1.4 kgf/cm.sup.2). Various
abrasive particles may be used for the grit-blasting, e.g., metal
oxide particles such as alumina, as well as silicon carbide, glass
beads, crushed glass, sodium carbonate, and crushed corn cob. The
average particle size usually is less than about 500 microns, and
typically less than about 100 microns.
The grit-blasting is carried out for a time period sufficient to
remove the degraded coating. The duration of grit-blasting in this
embodiment will depend on various factors. In the case of an
aluminide coating having a thickness of from about 50 microns to
about 100 microns, grit-blasting will usually be carried out for
from about 60 seconds to about 120 seconds, when utilizing an air
pressure of from about 20 psi (about 1.4 kgf/cm.sup.2) to about 30
psi (about 2.1 kgf/cm.sup.2) and grit particles having an average
particle size of less than about 100 microns.
Other known techniques for lightly abrading the surface may be used
in lieu of grit-blasting. For example, the surface may be manually
scrubbed with a fiber pad, e.g., a pad with polymeric, metallic or
ceramic fibers. Alternatively, the surface may be polished with a
flexible wheel or belt in which alumina or silicon carbide
particles have been embedded. Liquid abrasive materials may be used
on the wheels or belts. For example, they may be sprayed onto a
wheel in a vapor honing process. These alternative techniques can
be controlled to maintain a contact force against the substrate
surface that is no greater than the force used in the gentle
grit-blasting technique discussed above.
Other techniques may be employed to remove the degraded material.
One example is laser ablation of the surface. Alternatively, the
degraded material may be scraped off the surface. In another
embodiment, sound waves (e.g., ultrasonic waves), which may
originate from an ultrasonic horn, can be directed against the
surface to cause vibrations that can shake loose the degraded
material.
In some instances, the degraded coating may be removed by a more
aggressive agitation, e.g., agitation with a force greater than
that produced using the ultrasonic technique itself. For example,
the substrate can be immersed in a bath that is rapidly stirred
with a mechanical stirrer (i.e., for "general agitation"), and that
is also ultrasonically-stirred (i.e., for "local agitation").
Agitation can be carried out until the degraded material is shaken
loose. For each of these alternative techniques, those skilled in
the art would be familiar with operating adjustments that can be
made to control the relevant force applied to the substrate to
minimize damage to the substrate surface.
In some embodiments, an extended rinsing step may be used to remove
the degraded coating without damaging the substrate. This may
involve contacting the degraded aluminide coating with an aqueous
solution comprising a wetting agent like those described
previously, for example, a polyalkylene glycol such as polyethylene
glycol. The wetting agent is usually present at a level of from
about 0.1% to about 5% by weight, based on the total weight of the
rinsing solution. Rinsing can be carried out by a variety of
techniques, but is usually undertaken by immersing the substrate in
an agitated bath of the rinsing solution for a time period from
about 1 minute to about 30 minutes. The extended rinsing step can
remove chunks of aluminide particles and oxides from the substrate.
Any remaining thin layer of more coherent aluminide material may be
removed in another agitation step, or by again contacting the
substrate with the stripping composition.
In other embodiments, the degraded coating may be removed by
including the step of contacting the degraded coating with a
caustic material. The caustic may also clean the surface, remove
any surface oxides formed as a result of the stripping step, and
activate the surface for any additional processing steps, such as a
second stripping step. Examples of caustics include potassium
hydroxide (KOH), sodium hydroxide (NaOH), ammonium hydroxide
(NH.sub.4OH), lithium hydroxide (LiOH), triethylamine
((C.sub.2H.sub.5).sub.3N; TEA), tetramethylammonium hydroxide
((CH.sub.3).sub.4NOH; TMAH), and mixtures thereof. The contact time
can range from about 20 minutes to about 4 hours, although longer
or shorter times may be selected depending on the properties of the
particular caustic, coating and base metal.
The caustic may be in the form of a molten salt, but usually is
present as an aqueous solution comprising from about 10% to about
50%, typically from about 15% to about 30%, more typically from
about 17% to about 25%, of caustic, by weight of the composition.
The caustic solution usually has a temperature of from about
60.degree. C. to about 100.degree. C., typically from about
65.degree. C. to about 90.degree. C., more typically from about
70.degree. C. to about 85.degree. C.
The caustic solution may be applied to the substrate in a variety
of ways, but as described above, the substrate is typically
immersed in a bath of the caustic solution. In one embodiment, the
substrate is lowered into the bath using a suitable rack (for
example, one having a polypropylene or other non-conductive
surface) that can be raised to remove the substrate after the
desired immersion time is reached. The caustic solution is
typically agitated while in contact with the substrate. In one
embodiment, this is ultrasonic agitation. Alternatively, a more
aggressive agitation, such as described above, may be used.
A caustic solution such as described above may also be used to
pretreat, clean, or remove oxides from the metal-based substrate
prior to contact with the stripping composition. In one embodiment,
the caustic solution is used to clean or remove oxides from the
substrate prior to and after each contact with a stripping
composition. A rinsing step is typically provided between the
caustic and acid bath treatments herein to prevent potentially
violent reactions between the caustic and acid solutions.
After removal of the coating from the substrate, compressed air may
be blown across the substrate to remove any residual aluminide
particles, oxides, or abrasive particles. If desired, the substrate
can then be re-coated with any suitable material. For example,
platinum-aluminide protective coatings for engine parts can again
be applied to the surface of a superalloy substrate.
In some embodiments of the invention, the substrate surface may be
contacted with two (or more) stripping compositions, in sequence.
The first composition may quickly remove some of the aluminide
coating. The second (or subsequent) stripping composition may then
remove the remaining aluminide coating more slowly, with little or
no pitting or attack on the substrate except for the possible
occurrence of uniform corrosion, as discussed previously.
Typically, each stripping composition is present in the form of a
bath in which the substrate is immersed. Contact times and bath
temperatures may vary, as described previously. In one embodiment,
the substrate is immersed in a first bath maintained at a
temperature in the range of from about 4.degree. C. to about
12.degree. C., with an immersion time between about 3 and about 4
hours. After rinsing and contact with a caustic solution as
described above, the substrate is then immersed in a second bath,
typically also maintained at a temperature in the range of from
about 4.degree. C. to about 12.degree. C., with an immersion time
between about 3 and about 4 hours. Additional stripping steps may
be used, but are often unnecessary. As described above, the
substrate can then be subjected to various steps to remove the
degraded coating.
In another embodiment, the invention comprises a method for
selectively removing an aluminide coating from at least one
internal surface of a metal-based substrate, comprising the
following steps: (a) contacting the surface of the substrate with
an aqueous solution comprising from about 10% to about 50%, by
weight of the solution, of caustic at a temperature of from about
60.degree. C. to about 100.degree. C. for from about 1 to about 4
hours; (b) contacting the surface of the substrate with at least
one stripping composition comprising from about 20% to about 40%
nitric acid, by weight of the composition, at a temperature less
than about 20.degree. C. for at least about 2 hours to degrade the
coating without damaging the substrate; (c) contacting the surface
of the substrate with an aqueous solution comprising from about 10%
to about 50%, by weight of the solution, of caustic at a
temperature of from about 60.degree. C. to about 100.degree. C. for
from about 20 minutes to about 2 hours; (d) contacting the surface
of the substrate with at least one stripping composition comprising
from about 20% to about 40% nitric acid, by weight of the
composition, at a temperature less than about 20.degree. C. for at
least about 2 hours to degrade the coating without damaging the
substrate; and (e) contacting the surface of the substrate with an
aqueous solution comprising from about 10% to about 50%, by weight
of the solution, of caustic at a temperature of from about
60.degree. C. to about 100.degree. C. for from about 1 to about 4
hours to remove the degraded coating without damaging the
substrate.
In one embodiment, the caustic solution is ultrasonically agitated
while in contact with the substrate. The caustic solution typically
comprises from about 15% to about 30%, more typically from about
17% to about 25%, by weight of the composition, of caustic, such as
potassium hydroxide. The temperature of the caustic solution is
usually from about 65.degree. C. to about 90.degree. C., typically
from about 70.degree. C. to about 85.degree. C. In one embodiment,
the caustic solution contacts the substrate for a time ranging from
about 1.5 to about 2.5 hours in each of steps (a) and (e), and from
about 20 minutes to about 1 hour, typically from about 25 to about
35 minutes, in step (c).
In another embodiment, the method of the invention is used to
selectively remove the aluminide coating from the internal shank
and root surfaces of a high-pressure turbine blade. The shank
portion of the turbine blade typically is a high stress region
whose surfaces often operate below the ductile-to-brittle
transition temperature of the aluminide coating. This makes the
aluminide coating more susceptible to the formation of minute
cracks in the coating that can spread to the substrate and lead to
a metal fatigue and blade failure. In one aspect of the invention,
the aluminide coating is completely removed from the internal shank
surfaces, but not removed from the airfoil internal and external
surfaces where the protective coating is desired. This can be
achieved by immersing the turbine blade in the stripping
composition only up to the desired level of the shank portion of
the blade. This avoids the need for masking the airfoil portions of
the blade where the aluminide coating is desired. Since the
stripping composition is used at relatively low temperatures,
unlike mixtures of hydrochloric acid and nitric acid which are
typically heated to increase the stripping rate, acid fumes are not
produced that can attack unmasked areas of the airfoil internal and
external surfaces.
In another aspect, the invention relates to a turbine engine part
having a metal-based substrate and an aluminide coating on at least
one surface thereof, at least a portion of which coating has been
selectively removed from at least one surface of the substrate by a
method comprising the following steps: (a) contacting the surface
of the substrate with at least one stripping composition comprising
from about 20% to about 40% nitric acid, by weight of the
composition, at a temperature less than about 20.degree. C. for at
least about 2 hours to degrade the coating without damaging the
substrate; and (b) removing the degraded coating without damaging
the substrate.
In some embodiments, at least a portion of the aluminide coating
has been selectively removed from at least one surface of the
substrate by using the above method and selecting various stripping
compositions, caustics and processing conditions as described
above.
Another aspect of the present invention is directed to a method for
replacing a worn or damaged aluminide coating on at least one
surface of a metal-based substrate, comprising the following steps:
(i) selectively removing the aluminide coating from the surface of
the substrate by (a) contacting the surface of the substrate with
at least one stripping composition comprising from about 20% to
about 40% nitric acid, by weight of the composition, at a
temperature less than about 20.degree. C. for at least about 2
hours to degrade the coating without damaging the substrate; and
(b) removing the degraded coating without damaging the substrate;
and (ii) applying a new aluminide coating to the surface of the
substrate.
Techniques for applying the new aluminide coating are known in the
art. For example, various thermal spray techniques can be employed
for the deposition of overlay coatings. Examples include vacuum
plasma spray (VPS), air plasma spray (APS), and high velocity
oxy-fuel (HVOF). Other deposition techniques can be used as well,
such as sputtering and physical vapor deposition (PVD), e.g.,
electron beam physical vapor deposition (EB-PVD).
Various techniques are also known for applying diffusion coatings,
e.g., noble metal-aluminide coatings such as platinum-aluminide or
palladium-aluminide. As an example in the case of
platinum-aluminide, platinum can initially be electroplated onto
the substrate, using P-salt, Q-salt, or other suitable platinum
electroplating solutions. In a second step, the platinum layer is
diffusion-treated with aluminum vapor to form the
platinum-aluminide coating.
The following examples illustrate some embodiments of this
invention, but should not be construed to be any sort of limitation
on its scope. In the examples, each test sample was a high-pressure
turbine blade that had been used for some time in a commercial gas
turbine engine. The turbine blades were made from a single crystal
nickel-based superalloy, designated by the trade name Rene.RTM. N5.
The turbine blades were coated externally with a platinum-aluminide
bond coating and an EB-PVD yttria-stabilized zirconia thermal
barrier top coat. The internal surfaces of the turbine blades were
coated with a simple aluminide coating.
EXAMPLE 1
A sample blade was treated according to a process involving
multiple steps. First, the thermal barrier coating was aggressively
removed by grit blasting with aluminum oxide. The blade was then
injected with the commercial acid resistant Plastisol.RTM. resin to
protect the cooling holes and internal passages from the chemical
stripping solution. The blade was then immersed in a bath formed
from a 50:50 (by weight) mixture of nitric acid and phosphoric
acid. The bath was maintained at a temperature of about
170-190.degree. F. (about 77-88.degree. C.). After about 2 to 4
hours, the blade was removed and rinsed in cold tap water. The
Plastisol.RTM. resin was removed from the internal surfaces by
exposing the blade to a temperature of about 1100.degree. F. (about
593.degree. C.) for 1 hour in an air furnace. The external surface
of the blade was then lightly grit blasted with 220-mesh aluminum
oxide particles at a pressure of about 20-30 psi (about 1.4-2.1
kgf/cm.sup.2). The above process removed the aluminide coating from
the external surfaces of the blade, but not from the internal
surfaces.
EXAMPLE 2
A sample blade was treated using a process of the invention to
remove the aluminide coating from a portion of its internal
surfaces without damaging the external platinum-aluminide bond
coat. The process involved pre-treating and activating the surface
to be stripped, followed by immersion of the part in a cold nitric
acid solution to degrade the coating, and then immersion in hot
potassium hydroxide solution to remove the degraded coating. First,
the root plate from the trailing edge cooling circuit of the blade
was removed by grinding to provide better access to the internal
cooling passages of the blade. The blade surfaces were then
pre-treated and activated by immersion in an aqueous bath
comprising about 15% to 25% KOH (by weight) maintained at
160-180.degree. F. (about 71-82.degree. C.) for 2 hours with
ultrasonic agitation. After rinsing in de-ionized water, the blade
root and shank were immersed in a solution comprising 30% (by
weight) nitric acid and about 0.3% (by weight) Activol.RTM. 1658
wetting agent, maintained at 45-55.degree. F. (about 7-13.degree.
C.), for 3.5 hours to degrade the coating without damaging the
substrate. After rinsing in de-ionized water, the blade was
returned to the KOH bath for an additional 25-35 minutes, with
ultrasonic agitation to remove the degraded coating and re-activate
the surface. The blade root and shank were again immersed in the
cold nitric acid solution for 3.5 hours to degrade the remaining
coating without damaging the substrate. After rinsing in de-ionized
water, the blade was returned to the KOH bath for 2 hours with
ultrasonic agitation to remove the degraded coating.
The above process completely removed the aluminide coating from the
internal and external surfaces of the root and shank portions of
the blade with negligible effect on the base metal and on the
aluminide coating on the internal and external surfaces of the
airfoil portion of the blade. In contrast to Example 1, the process
avoided the need for masking the internal cavities of the blade to
prevent them from being blocked by grit particles, and also avoided
the use of a high-temperature stripping step that can attack the
base metal and generate acid fumes. The process sequence of
alternating cycles of exposure to hot KOH, rinse, cold nitric acid,
rinse, and hot KOH can be repeated a number of times to ensure
complete removal of the aluminide coating from the root and shank
portions of the blade.
EXAMPLE 3
A sample blade was treated using another process of the invention
to remove the aluminide coating from a portion of its internal
surfaces without damaging the external platinum-aluminide bond
coat. The process involved immersion of the part in a cold nitric
acid solution to degrade the coating and then immersion in hot
potassium hydroxide solution to remove the degraded coating. First,
the root plate from the trailing edge cooling circuit of the blade
was removed by grinding to provide better access to the internal
cooling passages of the blade. The blade root and shank were
immersed in a solution comprising 30% nitric acid (by weight) and
about 0.3% (by weight) Activol.RTM. 1658 wetting agent, maintained
at 35-40.degree. F. (about 2-4.degree. C.), for 5 hours to degrade
the coating without damaging the substrate. After rinsing in tap
water, the blade was immersed in an aqueous solution comprising
about 15% to 25% KOH (by weight) maintained at 160-180.degree. F.
(about 71-82.degree. C.) for 2 hours with ultrasonic agitation to
remove the degraded coating.
The above process also completely removed the aluminide coating
from the internal and external surfaces of the root and shank
portions of the blade, with negligible effect on the base metal and
on the aluminide coating on the internal and external surfaces of
the airfoil portion of the blade. These results were obtained even
though the process did not use the KOH pretreatment step used in
Example 2, and the nitric acid solution was maintained at a lower
temperature.
Various embodiments of this invention have been described. However,
this disclosure should not be deemed to be a limitation on the
scope of the invention. Accordingly, various modifications,
adaptations, and alternatives may occur to one skilled in the art
without departing from the spirit and scope of the claimed
invention.
* * * * *